Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes to convert solar energy directly into H2 is an elegant approach towards realizing storable and transportable clean energy resources. Although extensive research has been carried out since TiO2 was first used as a photocatalyst in the early 1970s, achieving high efficiency, operational stability and low-cost remain the major obstacles for its practical implementation. Here, we investigated several strategies to address the challenges impeding the PEC performance of TiO2.
First, we improved the optical absorption and charge transfer properties of TiO2 through morphological modification by preparing 3D ordered macroporous (3DOM) TiO2 films. The underlying interconnected TiO2 porous network improves light trapping efficiency, provides enhanced electrolyte/semiconductor interface and allows direct charge transfer pathways. Next, to improve the efficiency of TiO2, we investigated the effects of oxygen vacancies in these TiO2 3DOM films. The oxygen vacancies in TiO2 extended its light absorption, improved electrical conductivity and reduced charge carrier recombination. Lastly, TiO2 was coupled with a narrow band gap tantalum oxynitride (TaOxNy) to form a type II heterojunction 3DOM film, so as to improve the former’s overall water splitting performance. TaOxNy is an ideal candidate because it has band edges which straddles the redox potential of water and a tunable bandgap from 1.9 eV to 2.5 eV. From studies on layered doping in TaOxNy, we discovered photocurrent switching behavior, which is a promising development towards using the same material for both the half reactions of water splitting.
In this presentation, I will discuss the results of these works by presenting the analyses from material, optical, PEC and electrochemical impedance characterisations. I will specifically focus on the charge transfer mechanism between the 3DOM film and the electrolyte occurring at the semiconductor/electrolyte interface during PEC water splitting process.